This invention relates generally to combustors for use with gas turbines. More particularly, the invention provides a compact, low emission catalytic combustor for gas turbines operable in automotive and other environments.
Catalytic combustion makes possible extremely low emissions of pollutants from gas-powered turbine generators, particularly of oxides of nitrogen (NOx). A properly designed catalytic combustor can deliver both low NOx and low carbon monoxide (CO) emissions over the full engine operating range, in contrast to conventional combustors which may suffer from high NOx or high CO at different points in the duty cycle. In addition, stability and acoustic problems often associated with alternative low-NOx combustors are avoided with catalytic combustion, as are the complications of variable geometry.
Catalytic combustion is possible only when the combustor inlet temperature exceeds a minimum value that is a function of the catalyst formulation. This is typically about 700 F. Thus, a conventional diffusion-flame preheater is required for engine starting and for accelerating the engine to the speed necessary to obtain an adequate combustor inlet temperature. Once this condition has been reached, the preheater can be shut off. At this point a separate fuel delivery system is used to introduce fuel into the premix duct, where the fuel is evaporated (if liquid rather than gaseous fuel is used) and mixed with the incoming air. The resulting fuel-air mixture is then introduced into the catalyst bed. When liquid fuel is used, complete evaporation of the fuel as well as thorough mixing of air and the fuel must be achieved within the premix duct in order to obtain minimum combustor emissions and to avoid damage to the catalyst bed. Similarly, when gaseous fuel is used, thorough mixing of air and the fuel must be achieved within the premix duct.
Within the catalyst bed, combustion is initiated by catalytic action near the bed walls. Once initiated, combustion is continued by homogeneous combustion in the gas phase. Ignition by the catalyst makes possible complete combustion at very low flame temperatures, which results in extremely low NOx production. Support of the flame by the catalyst also results in high efficiency combustion, which leads to low CO emissions. Thus, both NOx and CO can be kept low over a wide range of engine speeds and loads.
While catalytic combustion is not a new technology, the present invention provides a relatively small-sized catalytic combustor compared to earlier examples of such combustors. For example, in previous catalytic combustor designs, the preheater has typically been designed to be remote from the premix duct. This is because the recirculating flows that are necessary to support diffusion-flame combustion within the preheater cannot be tolerated within the premix duct. At the relatively high combustor inlet temperatures necessary for catalytic operation, autoignition of the fuel within the premix duct is a distinct possibility. Excessively recirculating flows within the premix duct can lead to long residence times for the air-fuel mixtures in the duct and result in a high probability of autoignition. However, autoignition must be avoided in low emission combustion because autoignition can result in high flame temperatures and thus high NOx production. For this reason the preheater has in the past typically been physically separated from the premix duct.
The present invention, in addition to enabling the construction of a relatively small catalytic combustor, also addresses the problem of meeting or exceeding stringent emissions standards. For example, emissions standards for the Ultra-Low Emissions Vehicle (ULEV) specify stringent emission limits over a complete driving cycle from engine startup to shutdown. Thus, the present invention was designed to provide low emissions not only from the catalyst, but also from the preheater during its ignition through the engine spoolup and warmup phases to the transition to catalytic operation.